How can I add text in a non-english language?
First of all, Blender does not support Right-to-Left text rendering, however there are ways to use a wide variety of character sets.
In a 3D text object:
The default Bfont has a very limited character set which does not include arabic.
To add arabic text to a 3D text object, you need to use a font that has the relevant glyphs (e.g. a unicode font).
Once you find a font that contains arabic characters, you need to load it into blender.
Note that you cannot use bitmap fonts, as blender needs vector data for generating curves.
##Loading a font:
You can load a font into blender from the Object data &gt; Font panel:

Add a text object ShiftA&gt; Text

With the text object selected, click the file icon for the appropriate styling (e.g. regular, bold, etc.) in Properties &gt; Object Data &gt; Font:




Navigate to a font file (.ttf .otf etc.)  on your system and click Open Font:


The font should now be used for the active text object.
##Entering characters:
If you don't have a keyboard that has Arabic characters, here are some alternate ways to input text:
method one:

Select the text object and enter edit mode Tab. You should see a bar appear that indicates the location of the cursor:



Delete the existing characters (Backspace) and enter a character. You should see a text field appear in the Redo menu at the bottom of the Tool Shelf (T in the 3D view):



Select the text box and paste the your text into it, then press Enter:


Note that you can enable International fonts in User preferences &gt; System to display arabic charecters in the UI, making the arabic characters visible in the text field:

More about this later.
Method two:
As mentioned by kivig, there is a Paste from file feature.

Select the text object and enter edit mode (Tab)

Click Paste file in 3D view &gt; Header &gt; Text:




Navigate to a text file containing the text you wish to put in the text object and press Paste File.

In the Interface:
You can enable International Fonts in CtrlAltU&gt; User preferences &gt; System to display arabic characters:

This also enables options to translate the UI into many languages, including arabic. See the wiki for more details:

From version 2.60, Blender supports international fonts and a range of
language options for the Interface and Tooltips. To enable it, open
the User Preferences window, System tab, and toggle the International
Fonts option in the bottom right-hand corner.
This displays three new settings:

Language
drop-down menu where you can select your preferred language.

Interface
to translate the User Interface itself (e.g. controls and menus).

Tooltips
to translate tooltips.